python re.sub group: number after \number

How can I replace foobar with foo123bar?

This doesn’t work:

>>> re.sub(r'(foo)', r'\1123', 'foobar')
'J3bar'

This works:

>>> re.sub(r'(foo)', r'\1hi', 'foobar')
'foohibar'

I think it’s a common issue when having something like \number. Can anyone give me a hint on how to handle this?

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